U.S. Pat. No. 4,493,548 to Ateya, issued Jan. 15, 1985 shows an air transport device comparable to those in use today on high speed, single-pass duplex, duplicating machines. The machines for which they are designed create toner images on both sides of a receiving sheet before either image is fixed. To transport the receiving sheet from an image member to a fixing device without disturbing either loose toner image, the sheet is supported on a layer of forced air as it is pushed to the fixing device by the image member. This Unfused Copy Air Transport is called a UCAT. Its development permitted the simultaneous fixing of toner images on opposite sides of a sheet. Toner image-bearing sheets are suspended above a stationary surface by means of air flowing out through small ports in the surface. Airflow through the ports is supplied by a blower which maintains a suitable positive pressure inside a plenum chamber below the surface. The ports are angled and the surface featured in such a way as to push and pull on the sheet at the same time, effectively regulating the height of the sheet above the air plenum.
For double-pass duplex or for simplex applications, a vacuum transport is often employed. Sheets emerging from a transfer station are held against a moving belt as a result of negative pressure in a vacuum chamber behind the belt.
Positive air systems work well in the transport of paper but face a problem associated with interaction between upstream and downstream drive systems. Thus, when a sheet enters a faster moving fixing device nip, the air transport cannot resist and the sheet slips along it, tugging at the slower moving image member portion of the process. If a vacuum transport is used, the distance between the transfer and fixing stations can be made longer than the longest sheet. Although this may not permit a compact apparatus, it allows the fixing device to be run at any speed equal to or faster than the transfer station. If the entire drive for the sheet to the fixing device is supplied by the image member, the designer is forced to set the spacing between the image member and the fixing device at the shortest paper length and live with whatever harmful interactions exist for longer paper lengths.
U.S. Pat. No. 4,561,756 to Lang, issued Dec. 31, 1985, suggests speed mismatch compensation between the image member and a fixing device by allowing the sheet under the force of gravity to buckle against a set of baffles. The fusing device is deliberately run slightly slower than the image member to force the buckle in a direction controlled by the baffles. One of the baffles is grounded to attract the receiving sheet in a generally downward direction to help form the buckle.